An optical fiber may have an end surface provided with a dielectric film. A typical example of such a dielectric film is an anti-reflection film for preventing the end surface from reflecting light intended to enter the optical fiber.
A semiconductor laser module, for example, often includes an optical fiber having an end surface provided with such an anti-reflection film. A semiconductor laser module is a light source device for use as, for example, an excitation light source for a fiber laser or fiber amplifier, and includes a semiconductor laser element for generating laser light and an optical fiber for transmitting the laser light. A semiconductor laser module is arranged such that its optical fiber has a light entry end surface facing the light emission end surface of the semiconductor laser element, the light entry end surface being provided with an anti-reflection film so that laser light emitted by the semiconductor laser element enters the optical fiber efficiently.
A dielectric film has the property of adsorbing moisture present in the environment. In a case where a dielectric film is used as an optical component such as an anti-reflection film, it is important to prevent such moisture adsorption. This is because a dielectric film changes its optical property when it has adsorbed moisture. Patent Literatures 1 and 2, for example, each publicly disclose a technique for preventing such moisture adsorption.
Patent Literature 1 discloses a technique of mixing a fluorine compound in the top layer of an optical multilayer film (dielectric multilayer film) to prevent the optical multilayer film from adsorbing moisture. Patent Literature 2 discloses a technique of reducing the distance between particles of the film material of the top layer of a dielectric multilayer film to a distance smaller than a water molecule to prevent the dielectric multilayer film from adsorbing moisture.